Method of removing solid carbon dioxide

ABSTRACT

The invention provides a method of removing solid carbon dioxide from cryogenic equipment, including the steps of: (a) introducing a stream including ethane to the cryogenic equipment to convert solid carbon dioxide to liquid form whereby a mixture of liquid ethane and liquid carbon dioxide is formed; and (b) removing the mixture of liquid ethane and liquid carbon dioxide from the cryogenic equipment. In particular, the method can be used in a liquefied natural gas (LNG) plant wherein cryogenic equipment contains LNG, and the method includes the steps of: (a′) removing the LNG from the cryogenic equipment; (a) introducing a stream including ethane to convert solid carbon dioxide to liquid form whereby a mixture of liquid ethane and liquid carbon dioxide is formed; and (b) removing the mixture of liquid ethane and liquid carbon dioxide from the cryogenic equipment. The result is an effective cleaning method for fouled LNG equipment.

The present invention relates to a method of removing solid carbondioxide (CO₂) from cryogenic equipment, in particular from cryogenicequipment used in gas conditioning or gas deep-extraction processes, andmore particularly from cryogenic equipment used in the production ofLiquefied Natural Gas (LNG).

Natural gas contains a wide range of species which are capable offorming solids during the cryogenic process of producing LNG known asliquefaction. One of the species that causes considerable problems toLNG producers is carbon dioxide. In a conventional LNG facility,pretreatment of the natural gas is conducted to decrease the carbondioxide content to between 50 and 125 ppm prior to the natural gasentering the liquefaction process.

On average, carbon dioxide compositions in a natural gas feed stream canrange between 0.5% and 30% mol and can be as high as 70% mol incommercially viable reservoirs like Natuna, Indonesia. Carbon dioxide istypically removed using chemical reactions such as reversible absorptionprocesses with amine solvents. These are expensive and complex processesand commonly encounter operational problems such as foaming, corrosion,blocked filters and amine degradation. Losses of amine, water andhydrocarbons are commonly encountered. These processes also consumesignificant amounts of energy to regenerate and pump the solvent.

LNG refrigeration systems are expensive because so much refrigeration isneeded to liquefy natural gas. A typical natural gas stream enters a LNGplant at pressures from about 40 bar to about 76 bar and temperaturesfrom about 20° C. to about 40° C. Natural gas, which is predominantlymethane, cannot be liquefied by simply increasing the pressure, as isthe case with heavier hydrocarbons used for energy purposes. Thecritical temperature of methane is −82.5° C. This means that methane canonly be liquefied below that temperature regardless of the pressureapplied. Since natural gas is a mixture of gases, it liquefies over arange of temperatures. The critical temperature of natural gas istypically between about −85° C. and −62° C. Natural gas compositions atatmospheric pressure will typically liquefy in the temperature rangebetween about −165° C. and −155° C. Since refrigeration equipmentrepresents such a significant part of the LNG facility cost, cleaning ofthis equipment is important.

In conventional LNG plants, the natural gas is typically cooled in oneor more heat exchangers. If insufficient carbon dioxide is removed priorto the natural gas entering the heat exchangers, carbon dioxideprecipitates as a solid and accumulates on the cold surfaces of the heatexchangers and other plant equipment eventually rendering these itemsinoperable. When fouling has reached a critical level, the vessel mustbe taken off-line for the carbon dioxide solids to be removed. This canbe achieved by warming-up the affected equipment. However, this causesconsiderable downtime and energy-loss for the plant. Alternatively, thesolid carbon dioxide may be removed mechanically. In such a case ofmechanical defouling of equipment, the vessel, baffles and/or pipeworkmay be damaged, which only encourages further fouling in the nextproduction cycle. Moreover, solids condensing on metal surfaces form aninsulating film reducing the thermal efficiency of the heat exchanger.

There is a need for a simpler, more economical process for the removalof solid carbon dioxide that has fouled plant equipment under cryogenicconditions.

According to one aspect of the present invention, there is provided amethod of removing solid carbon dioxide from cryogenic equipment, themethod comprising the steps of:

-   (a) introducing a stream including ethane to said cryogenic    equipment to convert solid carbon dioxide to liquid form whereby a    mixture of liquid ethane and liquid carbon dioxide is formed; and-   (b) removing the mixture of liquid ethane and carbon dioxide from    the cryogenic equipment.

According to another aspect of the present invention, there is provideda method of removing carbon dioxide fouling of cryogenic equipmentcontaining LNG, the method comprising the steps of:

-   (a′) removing the LNG from the said cryogenic equipment;-   (a) introducing a stream including ethane to convert solid carbon    dioxide to liquid form whereby a mixture of liquid ethane and liquid    carbon dioxide is formed; and-   (b) removing the mixture of liquid ethane and liquid carbon dioxide    from the cryogenic equipment whereby the cryogenic equipment is    defouled of solid carbon-dioxide and available for the    reintroduction of the LNG stream.

Preferably the method comprises the step of adjusting the relativepercentages of ethane and carbon dioxide for a given pressure andtemperature such that the mixture of liquid ethane and carbon dioxide isnear azeotropic. By near azeotropic is understood a composition whereinthe carbon dioxide content varies between 5% mol below or above theazeotropic composition. It is known that ethane and carbon dioxide forman azeotrope. An azeotrope forms because of a particular molecularinteraction between two or more components. When such components aremixed, the vapour and liquid compositions at equilibrium are equalwithin a given pressure and temperature range. The formation of anazeotrope is usually seen to introduce an obstacle for the separation ofthe two components within the liquid mixture and is thus something thatis typically avoided in chemical processing plants. The azeotropiccomposition is dependent on the temperature and pressure, but isgenerally about 65% mol carbon dioxide and 35% mol ethane.

Although the stream including ethane may be gaseous, the streamincluding ethane is preferably liquid. This stream may contain pure orsubstantially pure ethane. It may also comprise other components.Preferably the stream including ethane contains at least 35% mol ethane.Suitable components include other hydrocarbons, such as propane andbutane, and carbon dioxide. It is preferred that the stream includingethane contains some carbon dioxide already. The fastest and mostefficient dissolution of carbon dioxide solids occur under theseconditions. The content of carbon dioxide may suitably vary from 0 toabout 65% mol.

Preferably, the method further comprises the step of separating themixture of liquid ethane and carbon dioxide to form a first product richin ethane and a second product rich in carbon dioxide. More preferably,the first product rich in ethane is available for recycling to step (a).More preferably, the second product rich in carbon dioxide is alsorecovered and recycled. When the carbon dioxide content is somewhatlower than the azeotropic composition the separation between anethane-rich product and a carbon dioxide-rich product can easily beachieved by distillation. The ethane-rich product may then besubstantially pure ethane, whereas the carbon dioxide-rich product hasthe azeotropic composition.

When the mixture of liquid ethane and carbon dioxide has an azeotropiccomposition, the method of separating the azeotropic mixture may includedistillation or membrane-based separation techniques or a combinationthereof. The method may include the step of introducing one or morealkanes to the azeotropic mixture prior to the separation step. Theaddition of one or more alkanes has the effect of widening the two-phaseliquid vapour equilibrium area in the ethane-carbon dioxide system toallow easier separation of ethane and carbon dioxide from the azeotropicethane-carbon dioxide liquid mixture.

Preferably, the cryogenic equipment is selected from the list includingheat exchangers, pipes, storage vessels, sub-cooling vessels and/orseparators.

The present invention will now be described in more detail by means thefollowing example.

The present invention derives from observations made during a series oftests conducted using a cryogenic vessel known as the Sapphire Cell. TheSapphire Cell as the name suggests is constructed of pure single crystalsapphire and allows hitherto impossible direct observation of thephenomena occurring during LNG liquefaction. Based on their observationsof these phenomena, the applicant has realised that liquid ethane can beused to remove carbon dioxide fouling of cryogenic equipment.

During testing, the Sapphire Cell was used as a flash vessel in fluidcommunication with a cryogenic chamber. Natural gas was introduced tothe Sapphire Cell and flashed down to 40 bar at −82° C. whereby LNG wasformed. Under the conditions at which liquefaction takes place, thecarbon dioxide still present in the natural gas feed stream willprecipitate out in solid form within the flash vessel.

The LNG produced was stored in the cryogenic chamber and the system wascooled down to −80° C using a multi-component refrigerant system andwith liquid nitrogen down to −161° C. The cryogenic chamber wasmaintained at the same pressure as the flash vessel until equilibriumconditions were attained so that the vapour-liquid equilibrium phasediagrams for a given range of compositions could be generated.

The liquid level within each of the flash vessel and cryogenic chamberwas measured using simple volumetric calibration. The liquid levelwithin the Sapphire Cell could also be observed by the eye through thetransparent walls of the Cell.

The temperature of the system was monitored using temperature sensorsinside each of the chambers with a third temperature sensor monitoringthe air bath around the cryogenic chamber and flash vessel. Pressuresensors were located outside the air bath at the inlet and outlet ofeach of the cryogenic chamber and the flash vessel. Multi-port samplingvalves were provided for each of the cryogenic chamber and flash vesselto allow on-line gas chromatographic analysis of samples when desired.

The system was agitated using a vortex operated magnetically until solidseparation of the carbon dioxide was observed. The vortex encouragedgravity separation of more dense carbon dioxide solids to the bottom ofthe chamber. The effect of creating a vortex is to draw the solidsformed within the vessel towards the wall of the vessel where theymigrate down towards the bottom of the vessel. The vortex can beestablished by mechanical means using a stirrer or by including ahydrocyclone at the base of the vessel.

In the first series of tests natural gas of known composition asoutlined below in Table 1 was introduced through a control valve intothe Sapphire Cell. TABLE 1 GC Analysis of the Feed Gas* Component MoleFraction 1 N₂ 2.54 2 CO₂ 2.39 3 C₁ 84.16 4 C₂ 7.08 5 C₃ 3.05 6 iC₄ 0.317 nC₄ 0.38 8 iC₅ 0.05 9 nC₅₊ 0.04*(Gas includes ppm's of mercaptan)

In a second series of tests, additional carbon dioxide was added to thechamber to bring the carbon dioxide content up to 25% as outlined belowin Table 2. TABLE 2 GC Analysis of the Feed Gas with Addition of ExtraCO₂* Component Mole Fraction 1 N₂ 1.939 2 CO₂ 24.95 3 C₁ 64.64 4 C₂5.493 5 C₃ 2.385 6 iC₄ 0.239 7 nC₄ 0.292 8 iC₅ 0.038 9 nC₅₊ 0.023*(Gas includes ppm's of mercaptan)

LNG was transferred into the cryogenic storage vessel leaving behind aslush comprising a relatively small percentage of LNG plus solid carbondioxide crystals in the flash vessel. The composition of the LNGproduced during the liquefaction process is outlined below in Table 3.From this table it can be seen that the carbon dioxide composition hasbeen reduced from 25% in Table 2 to just 0.29% due to the freeze out ofcarbon dioxide solids. TABLE 3 GC Analysis of the Produced LNG After CO₂Separation at 10 bar −140° C. 1 N₂ 1.28 2 CO₂ 0.29 3 C₁ 94.65 4 C₂ 4.485 C₃ 2.02 6 iC₄ 0.21 7 nNC₄ 0.27 8 iC₅ 0.04 9 nC₅₊ 0.03Carbon dioxide content was increased to 30% in the original gascomposition shown in Table 1.

The flash vessel containing the slush was left for one hour to achieveequilibrium. Liquid ethane was introduced at the same conditions of −8°C. and 26 bar. It was observed that the solid carbon dioxide within theslush started to dissolve immediately as liquid ethane was introduced.

In a third series of tests, a liquid mixture of 15% mol carbon dioxideand 85% mol ethane was introduced into the Sapphire Cell. The contentsof the Sapphire Cell were agitated using the magnetically inducedvortex. The transparent walls of the Sapphire Cell made it possible toobserve the fine solid crystals of carbon dioxide forming and dissolvingin rapid succession.

In the first referred embodiment of the present invention, a heatexchanger or other cryogenic pipework fouled with carbon dioxide istaken off-line. Liquid ethane is then introduced into the heat exchangeror pipework. The carbon dioxide solids dissolve as they convert back toliquid form. Dissolution of carbon dioxide occurs at any composition ofthe stream including ethane. The fastest rate of dissolution has beenobserved to occur when the stream includes ethane and carbon dioxide,and in particular when the ethane and carbon dioxide are present in suchstream in sufficient relative amounts at a given pressure andtemperature to form an azeotropic mixture. Under azeotropic conditions,dissolution of the carbon dioxide solids is observed to happen at itsgreatest speed and with greatest efficiency.

Having introduced the ethane and converted the solid carbon dioxide toliquid form, it is preferable for the mixture of ethane and carbondioxide to be separated to recover and recycle the ethane.

The most common method of separating homogeneous liquid mixtures is theuse of distillation, i.e. repeated vaporisation and condensation wherebythe vapour phase gradually becomes enriched in the more volatilecomponent. However, separation of a liquid mixture by distillationdepends on the fact that even when a liquid is partially vaporised, thevapour and liquid compositions differ. The vapour phase becomesprogressively more enriched in the more volatile component and isdepleted in the less volatile component. Repeated partial vaporisationis used to achieve the desired degree of separation. An azeotrope,however, cannot be separated using ordinary distillation since littleenrichment of the vapour phase occurs with each partial vaporisationstep.

Therefore in most cases, azeotropic liquid mixtures require specialmethods to facilitate separation of the component species.

Separation of the azeotropic mixture may be effected using techniquessuch as extraction, absorption, crystallisation, decanting, multi-stageextraction or other chemical treatments or any combination thereof. Inorder to use extractive distillation in either a continuous or batchoperation, it may be necessary to add an entrainer such as propane,butane or other suitable alkane or a combination thereof, the choicebeing dependent on the particular phase behaviour of the system andavailable compounds. It is envisaged that the alkane or alkanes would berecovered and recycled to the system also.

Alternatively, membrane separation methods may be used prior to orindependently of distillation. Such methods include dialysis, reverseosmosis, ultra-filtration, electrodialysis, helium separation throughglass, hydration separation through palladium and alloy membranes,immobilised solvents and/or liquid-surfactant membranes. The drivingforce for separation using membranes is either a pressure orconcentration difference across the membrane. Membranes may be used tobreak azeotropic mixtures prior to feeding the mixture to a subsequentcontinuous or batch distillation separation process.

In the second preferred embodiment of the present invention, the methodcan be used for removing solid carbon dioxide from cryogenic equipmentused in the production of LNG. The LNG would first be drained from thesystem before introducing liquid ethane in the manner outlined above.

A series of tests conducted using the Sapphire Cell have confirmed thatthe presence of methane in the natural gas feed stream has little or noeffect on the formation of carbon dioxide solids during LNG liquefactionnor the subsequent dissolution of the carbon dioxide solids when theethane is introduced.

It is proposed that this method of removing carbon dioxide contaminantscould be used for pipelines for carrying LNG, heat exchangers, cryogeniccooling vessels, and any other plant equipment used under cryogenicconditions where carbon dioxide fouling occurs.

It will be readily apparent to a person skilled in the relevant art thatthe present invention has significant advantages over the prior artincluding, but not limited to, the following:

-   (a) Existing LNG plants can be defouled without any requirement for    modification of the plant equipment;-   (b) Recycling of the ethane will significantly contribute to    reducing the cost of applying the method according to the present    invention for the removal of the carbon dioxide solid contaminants;-   (c) The process is applicable to a wide variation of feed gas    compositions; and-   (d) The carbon dioxide content of the natural gas can be adjusted in    order to assist in the removal of the carbon dioxide solids by the    ethane.

1. A method of removing solid carbon dioxide from cryogenic equipment,comprising the steps of: (a) introducing a stream including ethane tosaid cryogenic equipment to convert solid carbon dioxide to liquid formwhereby a mixture of liquid ethane and liquid carbon dioxide is formed;and (b) removing the mixture of liquid ethane and liquid carbon dioxidefrom the cryogenic equipment.
 2. The method of claim 1, in which thecryogenic equipment is used to produce liquefied natural gas.
 3. Themethod of claim 1 further comprising the step of adjusting the relativepercentages of ethane and carbon dioxide for a given pressure andtemperature such that the mixture of liquid ethane and liquid carbondioxide is near azeotropic.
 4. The method of claim 1 in which the streamincluding ethane contains carbon dioxide up to 65% mol.
 5. The method ofclaim 1 in which the method further comprises the step of separating themixture of liquid ethane and liquid carbon dioxide to form a firstproduct enriched in ethane and a second product enriched in carbondioxide.
 6. The method of claim 5, in which the first and second productare separated by distillation, extraction, absorption, crystallisation,decanting, multi-stage extraction or other chemical treatments or anycombination thereof.
 7. The method of claim 1, in which the mixture ofliquid ethane and liquid carbon dioxide is azeotropic, and is separatedto form a first product enriched in ethane and a second product enrichedin carbon dioxide by extractive distillation or membrane-basedseparation techniques or a combination thereof.
 8. The method of claim5, in which one or more alkanes or their isotropes are introduced to themixture prior to the separation step.
 9. The method of claim 5, in whichthe stream that includes ethane comprises the first product that isrecycled to the step (a).
 10. A method of removing solid carbon dioxidefrom cryogenic equipment, wherein the cryogenic equipment containsliquefied natural gas, the method comprising the steps of: (a′) removingthe liquefied natural gas from the said cryogenic equipment; (a)introducing a stream including ethane to convert solid carbon dioxide toliquid form whereby a mixture of liquid ethane and liquid carbon dioxideis formed; and (b) removing the mixture of liquid ethane and liquidcarbon dioxide from the cryogenic equipment.
 11. A method of producingliquefied natural gas, wherein natural gas is introduced in cryogenicequipment and is cooled down to form liquefied natural gas, and furtherwherein solid carbon dioxide is removed from the cryogenic equipment by:(a) introducing a stream including ethane to the cryogenic equipment toconvert solid carbon dioxide to liquid form whereby a mixture of liquidethane and liquid carbon dioxide is formed; and (b) removing the mixtureof liquid ethane and liquid carbon dioxide from the cryogenic equipment.12. The method of claim 11, wherein relative percentages of ethane andcarbon dioxide are adjusted for a given pressure and temperature suchthat the mixture of liquid ethane and liquid carbon dioxide is nearazeotropic.
 13. The method of claim 11, wherein the stream includingethane contains carbon dioxide up to 65% mol.
 14. The method of claim11, wherein the liquefied natural gas is removed from the cryogenicequipment prior to the step (a).
 15. The method of claim 2, furthercomprising the step of adjusting the relative percentages of ethane andcarbon dioxide for a given pressure and temperature such that themixture of liquid ethane and liquid carbon dioxide is near azeotropic.16. The method of claim 2, in which the stream including ethane containscarbon dioxide up to 65% mol.
 17. The method of claim 3, in which thestream including ethane contains carbon dioxide up to 65% mol.
 18. Themethod claim 2, in which the method further comprises the step ofseparating the mixture of liquid ethane and liquid carbon dioxide toform a first product enriched in ethane and a second product enriched incarbon dioxide.
 19. The method claim 3, in which the method furthercomprises the step of separating the mixture of liquid ethane and liquidcarbon dioxide to form a first product enriched in ethane and a secondproduct enriched in carbon dioxide.
 20. The method claim 4, in which themethod further comprises the step of separating the mixture of liquidethane and liquid carbon dioxide to form a first product enriched inethane and a second product enriched in carbon dioxide.